1. Field of the Invention
The present invention relates to an injection molding machine having a screw provided with a check ring.
2. Description of the Related Art
There is conventionally used an injection molding machine having an injection mechanism whose screw provided at its tip end with a check ring mechanism so as to prevent backflow of resin at the time of injection like an in-line screw type injection molding machine. FIG. 1 shows one example of the check ring mechanism. The screw 1 is provided at its tip end with a screw head 2. A portion of the screw 1 which is reduced in diameter is provided between the screw head 2 and a main body of the screw 1. A check ring 3 is provided at the diameter-reduced portion such that the check ring 3 can move in an axial direction of the screw. A check seat 4 is provided at the diameter-reduced portion and on the side of the main body of the screw 1. The check seat 4 comes into intimate contact with the check ring 3 to close a resin passage.
A resin pellet is supplied from a rear portion of the screw 1. The resin pellet is melted by shearing heat generated when the screw 1 rotates and by heat from a heater provided outside a cylinder into which the screw 1 is inserted. The molten resin increases a resin pressure behind the check ring 3 and generates a force for pushing the check ring 3 forward. If the check ring 3 is pushed forward, the resin located behind the check ring 3 passes through a gap between the check ring 3 and the diameter-reduced portion and flows forward of the check ring 3, and increases a pressure in the cylinder in front of the screw head 2.
If a pressure in front of the check ring 3 exceeds a predetermined pressure (back pressure), the screw 1 is pushed backward, and the pressure in front of the check ring 3 is reduced. If the screw 1 rotates further, a pressure behind the check ring 3 becomes higher than the pressure in front of the check ring 3 and thus, resin which is continuously melted is sent forward of the check ring 3, and if the screw 1 is retreated by a predetermined amount, the rotation of the screw 1 is stopped (metering step).
Next, the procedure is advanced to an injecting step. If the screw 1 moves forward to charge resin, a resin pressure accumulated in front of the screw head 2 is increased and thus, the check ring 3 is retreated and comes into intimate contact with the check seat 4 to close the resin passage, and the charging pressure prevents backflow of the molten resin in the retracting direction of the screw. If the check ring 3 retracts and timing for closing the resin passage is varied, an amount of resin to be charged is also varied and the molding becomes unstable.
The check ring mechanism at the time of injection is closed when a pressure in front of the check ring 3 becomes higher than a pressure behind the check ring 3 due to forward movement of the screw 1. However, there is a problem that a pressure remains a groove 6 between flights 5 and the screw 1 behind the check ring mechanism immediately before injection, and the closing point in time is varied due to the remaining pressure. Backflow of resin is generated from the front portion toward the rear portion of the check ring until the check ring is closed after the injection is started. Therefore, the injection capacity is varied between cycles due to the variation of the closing point in time and as a result, this affects quality of a molded article. Hence, there is conceived means for closing the check ring 3 in stable timing every cycle, and there is proposed a method for monitoring the time at which the check ring 3 is actually closed.
(a) For example, Japanese Patent Applications Laid-open No. 4-53720 and No. 4-201225 disclose techniques in which a pressure sensor for detecting a resin pressure in the cylinder is provided at a location rearward from the check ring, the closing motion of the check ring is detected based on a pressure variation detected by the pressure sensor during forward movement of the screw, and it is determined whether the quality of a molded article is non-defective or defective and the molding condition is adjusted based on the detected close position of the check ring.
In the techniques described in the above patent documents, the pressure variation in the cylinder is detected to detect the close of the check ring. According to this method, it is necessary to dispose the pressure sensor behind the check ring. Further, it is necessary that the pressure sensor is disposed away from a tip end of the cylinder by at least the maximum injecting stroke. Therefore, a distance between the check ring and the pressure sensor is varied depending upon the magnitude of the injection stroke, and the detection precision is varied. It is preferable that an inner wall surface of a cylinder is not provided with a step and a smooth passage is formed so that carbide is not produced by the remaining resin. In this case, if a pressure sensor which comes into direct contact with resin is mounted, it is inevitable that a fine step is generated on inner surface of a cylinder, such an adverse influence that a carbide caused by remaining resin is mixed into a molded article can not be avoided. Further, since a distortion of a cylinder is indirectly detected without direct contact with resin, the detection precision is deteriorated in the case of a pressure sensor which detects a resin pressure. There are problems that any type of pressure sensor is expensive and its handling is troublesome, periodical maintenance and calibration are required.
(b) Japanese Patent Application Laid-open No. 3-92321 discloses a technique in which a conductive member is disposed in the check ring such as to be opposed to the ring valve rearward of the ring valve, and a position of the ring valve (e.g., closing point in time of the resin passage by the ring valve) is detected by detecting capacitance between the ring valve and the conductive member.
According to the technique for detecting the closing point in time of the ring valve by detecting the capacitance as described in the above patent document, there are drawbacks that the conductive member for detecting the capacitance must be disposed in the screw, a hole through which a wire passes must be formed in a center of the screw, a slip ring for taking out a measurement signal must be disposed in the screw and means for measuring the capacitance must be added, and the structure becomes complicated.
(c) Japanese Patent Application Laid-open No. 1-168421 discloses a technique in which, although detection of closing point in time of the check ring during injection is not disclosed, a rotation torque acting on the screw at the time of injection is detected, and abnormality such as damage of the check ring is detected based on this detected torque.
(d) Japanese Patent Application Laid-open No 2004-216808 discloses a technique in which if the screw is brought into its rotatable state and injection is started, then resin flows backward and the screw is rotated, but the check ring is closed and if the backflow of resin is stopped, the rotation of the screw is stopped, and this fact is utilized and the stop of rotation of the screw is detected as the closing point in time of the check ring, and a switching position of the injection speed and a switching position to the hold pressure are corrected based on the detected closing position of the check ring.
In the technique described in the above patent document, the stop of rotation of the screw which is rotatable during injection is detected, and then the close of the check ring is detected. As shown in FIG. 2, when backflow is generated, a force F acts on the flight 5 of the screw 1 by the backflow of resin. In a normal molding machine, a sensor for detecting a resin pressure is located rearward of the screw (rightward of the screw in the drawing paper), but this sensor detects a pressure acting in the axial direction of the screw with respect to the screw. Therefore, a resin pressure in front of the screw head is also applied to the sensor, and it is difficult to detect a force Fx in the axial direction component of the force flown-back resin acts on the flight.
According to the technique described in the above patent document, attention is paid to a component force Fθ in the rotation direction of the screw of a force flown-back resin acts on the flight, the stop of rotation of the screw which is rotatable during injection is detected and the close of the check ring is detected.
When the rotatable screw is rotated by the backflow resin, however, if the backflow amount is small, since a force for rotating the screw by the backflow resin is within a range of the maximum static friction force with respect to the cylinder, the screw does not rotate. If the backflow amount is increased and the force for rotating the screw exceeds the maximum static friction force, the screw starts rotating. If the screw once starts rotating, the state thereof is shifted to a dynamic friction region, the screw keeps rotating if the force for rotating the screw is greater than the dynamical friction force even if the force for rotating the screw by the backflow resin becomes lower than the maximum static friction force. Therefore, when the force for rotating the screw by the backflow resin is greater than the dynamical friction force and lower than the maximum static friction force, if the rotation of the screw has already been stopped, the stopped state continues, and if the screw was rotating, the rotating state is continued. Hence, it can not be said that a linear relation always exists between the magnitude of the backflow amount and the magnitude of the rotation amount. Therefore, in the case of the method for detecting the close of the check ring from the rotation amount of the screw as in the technique described in the above patent document, there is a possibility that the detection of the closing point in time includes an error.
In the technique described in the above patent document, the fact that the rotation of the screw is stopped is detected, but it is necessary to determine a threshold value for detecting the stop of the rotation. The rotation of the screw may be stopped gradually or abruptly. In order to precisely detect the stop of the rotation under such various conditions, it is necessary to appropriately set the threshold value. However, to obtain an appropriate threshold value, a number of steps are required, and if molding state is changed, the threshold value must be adjusted again in some cases.
(e) Japanese Patent Application Laid-open No. 2006-69219 discloses a technique in which a screw is reversely rotated after metering is completed, a backflow preventing mechanism for interrupting a metering portion and a front of a screw head is provided, and after metering is completed, the screw is reversely rotated, the metering portion and the front of the screw head are interrupted by the backflow preventing mechanism and then, injection is started, and it is determined whether a molded article is non-defective or defective based on a stroke position or stroke amount at the time of injection.
The technique described in the above patent document has a problem that the special backflow preventing mechanism which interrupts the metering portion and the screw head by reversely rotating the screw after the metering is completed is required, and this technique can not be applied to a molding machine having a general backflow preventing mechanism.